The present invention relates to an imaging system wherein an object in an object plane is illuminated by an elongated light source and the reflected image is transmitted to an imaging plane by a gradient index lens array. More particularly, it relates to a gradient index lens array modified so as to provide a spatially uniform exposure level at the image plane.
Image transmitters comprising bundled gradient index optical fibers are known in the art. U.S. Pat. No. 3,658,407 describes a light conducting fiber made of glass or synthetic resin which has a refractive index distribution in a cross section thereof that varies parabolically outward from a center portion thereof. Each fiber acts as a focusing lens to transmit part of an image of an object placed near one end. An assembly of fibers, in a staggered two-row array, transmit and focus an image of the object. The fiber lenses are produced under the trade name "SELFOC"; the mark is registered in Japan and owned by Nippon Sheet Glass Company, Ltd.
Numerous techniques are known in the art for manufacturing glass or plastic fibers with index-of-refraction variations. These are usefully summarized in an article entitled "Gradient Index Optics: A Review" by Duncan T. Moore, Applied Optics, Apr. 1, 1980, Volume 19, No. 7, pages 1035-1038. Relevant optical characteristics of gradient index lens arrays are described in an article entitled "Some Radiometric Properties of Gradient-Index Fiber Lenses", by James D. Rees and William Lama, Applied Optics, Apr. 1, 1980, Volume 19, No. 7, pages 1065-1069.
Gradient index lens arrays have found use in a number of technologies, e.g. in construction of printed type optical circuits as disclosed in U.S. Pat. No. 3,922,062 and as a replacement for conventional optical systems in copiers as disclosed in U.S. Pat. Nos. 3,947,106 and 4,193,679.
In copier applications such as those disclosed in the referenced patents, the light source which provides illumination of the document to be copied must be able to provide an illumination band which is quite narrow and intense relative to the illumination band required for copiers using conventional projection lenses. This requirement is necessitated by the inherent operational structure of the gradient index lens array. Another requirement, common to all copier optical systems, is that given a uniformly bright object, a uniform level of image exposure be provided at the image plane. A typical cause of non-uniform image plane exposure is the light fall-off at the ends of elongated tubular sources such as fluorescent lamps. This light fall-off produces a non-uniform illumination of the document scan line, the nonuniformity being transmitted through the lens array to create a corresponding non-uniform image exposure.
Various techniques have been devised to compensate for this non-uniformity. The light source may be modified to produce a uniform level of document illumination by using a longer tube so that only the central portion of the tube provides the useful illumination. This, however, is a relatively inefficient expedient which increases the size of the copier. Another technique, borrowed from conventional lens systems, is to introduce a light shaping component such as a variable density filter or a variable aperture slit in the optical path, generally just before the imaging plane. The slit is appropriately shaped to permit more illumination to pass through the ends than in the center, i.e. the well known "butterfly slit" while a filter is denser in the middle and increasingly transparent at the ends. However, because of the much narrower ray bundle which is projected by the gradient index array, these techniques are exceedingly difficult to implement with any degree of accuracy.
The present invention provides a novel and relatively simple way to achieve a uniform level of image plane exposure without requiring the use of specially designed lamps or additional light shaping components. The compensation for end fall-off is achieved by altering the nature of the gradient lens array in such a way that greater illumination is transmitted at the end portion than at the central portion. This alteration is achieved by varying any one of the lens parameters such as the fiber packing density, individual fiber radius or individual fiber index gradient. Given a particular illumination profile which has end non-uniformity characteristics, a lens array can be designed according to the invention whereby an exposure distribution that is spatially uniform can be achieved at the image plane.